ROSAT observations of a soft X-ray emission component in the intermediate polar RE 0751+14

Abstract

ROSAT pointed observations of the intermediate polar (IP) RE 07 51 + 14 reveal, for the first time in an IP, a distinct soft X-ray component reminiscent of that seen in the strongly magnetic polars. The soft X-ray light curve, which is modulated on the 13.9- min spin period of the white dwarf, is characterized by a large-amplitude, quasi-sinusoidal variation, the rising phase of which is cut into by a dip feature with a deep, narrow core. In conjunction with the results of a more complete analysis of the previous Ginga hard X-ray observation of the star, the energy dependence of this dip suggests that it is caused by absorption, probably arising from the passage of dense, accreting material in front of the emission region. The presence of the dip feature in both the ROSAT and Ginga light curves is used to derive a rotational ephemeris. Phase-resolved spectral analyses of the separate Ginga and ROSAT data sets are presented. When combined, spectral fits indicate that models involving partial covering are, in general, a better description of the observed spectra than simple absorber models. We find no clear evidence of a gross imbalance between the luminosities of the hard and soft spectral components in RE 0751 + 14. We discuss the implications of these results on our understanding of this particularly important binary which, despite being an intermediate polar, exhibits several properties that are more characteristic of the AM Her stars.